In a related lighting device, it has been widespread to cause a light source such as (a) light emitting diodes (LEDs) to go on. In the field of lighting devices configured to cause vehicle headlights to go on, headlight devices equipped with LEDs and the like as light sources have been mass-produced.
A conventional headlight device may include a cooling fan for cooling a light source. Such a conventional headlight device drives the fan to increase thermal diffusion effect, thereby suppressing an increase in temperature caused by heating of the light source. The fan may however decrease a speed of rotation thereof or stop rotating due to aged deterioration of the fan, and the like. The fan decreasing the speed of rotation or stopping rotating may cause a malfunction of the headlight device as a result of an increase in temperature of the light source.
A headlight device disclosed in JP 2010-153343 A (hereinafter referred to as “Document 1”) is configured to receive, from a fan, a pulse signal synchronized with a speed of rotation of the fan and detect a rotation malfunction of the fan when a high or low level duration (pulse width) during one cycle of the pulse signal is predetermined time or more. In the headlight device of Document 1, when a rotation malfunction of the fan is detected, a control circuit of the headlight device stops the supply of electric power to a light source and the fan.
In a lighting device like Document 1, a binary signal such as a pulse signal is employed as a rotation detection signal representing a speed of rotation of a fan.
In a related art such as Document 1, there is a possibility that a rotation malfunction will be detected in error owing to instantaneous fluctuation of a binary rotation detection signal.